1. Field of the Invention
The present invention relates to a surface acoustic wave device used as a high-frequency oscillator in telecommunication equipment as well as to a method for fabricating the device.
2. Description of Related Art
FIG. 10 is a cross-sectional view of a conventional surface acoustic wave device. In FIG. 10, reference numeral 1 denotes a piezoelectric substrate (called the chip hereunder) constituting a surface acoustic wave element. On a principal plane of the chip 1 (upper surface in FIG. 10) are a surface acoustic wave functional portion la and electrode pads (the latter not shown). The functional portion 1a comprises driving electrodes made of known finger-shaped inter-digital transducers (called IDTs hereunder), and propagation paths for surface waves driven in a predetermined direction on the chip surface. The electrode pads extend from the driving electrodes and serve as external connection terminals of the functional portion 1a. Reference numeral 12 denotes a package that holds the chip 1, and 12a and 12b denote package side walls that surround the chip thereby constituting a chip holder. Part of the side wall 12a includes a terminal portion 12c serving as external connectors. Reference numeral 4 denotes wires for electrically connecting the electrode pads of the functional portion 1a to the terminal portion 12c.
Reference numeral 11 denotes a metal cover connected to a sealing portion 12d placed on the top surface of the package side wall 12b, whereby the functional portion 1a of the chip 1 is sealed in an airtight and protected 30 manner. In order to release acoustically both the surface waves driven by the known IDTs and the propagation paths for the waves, the surface acoustic wave device requires that a hollow portion be secured over the surface of the functional portion 1a and that protective measures be taken to prevent breakdown of the functional portion 1a.
FIG. 11 is a cross-sectional view of another conventional surface acoustic wave device disclosed in Japanese Patent Application Laid-open No. (Hei)4-301910. In FIG. 11, those parts with their counterparts already shown in FIG. 10 are given the same reference numerals, and descriptions of such parts are omitted where they are repetitive.
The device in FIG. 11 has the functional portion 1a furnished at the bottom of the chip 1, with a hollow portion formed between the functional portion 1a and the package 12. Bump electrodes 6 connect the functional portion 1a with the terminal portion 12c. That is, electrical connections between the portions 1a and 12c are secured by connecting the bump electrodes 6, formed on electrode pads not shown, to the terminal portion 12c. The package side wall 12a creates a staggered portion opposite to the functional portion 1a and also contributes to providing the hollow portion 9. The setup protects the functional portion 1a and secures a clearance over its surface. As in the case of the device in FIG. 10, the metal cover 11 on top of the chip 1 seals the opening of the package 12 airtight with the sealing portion 12d interposed therebetween.
Conventional surface acoustic wave devices are typically constituted as outlined above. In such devices, the cover 11 made of metal has been used for airtight sealing. When an alternative insulating resin, although less expensive, is used to surround the chip 1 periphery, the resin adheres on the chip 1 and makes it impossible to secure a hollow portion over the surface of the functional portion 1a. Meanwhile, the use of the metal cover pushed up manufacturing costs of the surface acoustic wave device, a disadvantage that has plagued the prior art.
It is therefore an object of the present invention to overcome the above and other deficiencies of the prior art and to provide a surface acoustic wave device that is small, lightweight, highly reliable, and is protected against breakdown of its functional portion with a hollow segment secured over the surface of that functional portion.
It is another object of the present invention to provide a method for fabricating a surface acoustic wave device in ways that are simpler, more efficient, more accurate and less expensive than before.